Preparation of bidentate ligands

ABSTRACT

A process is disclosed for preparing bidentate ligands of the formula: ##STR1## wherein: each AR is independently selected from aromatic ring compounds having 6 up to 14 carbon atoms, e.g., phenyl, naphthyl, phenanthryl and anthracenyl; 
     the x bonds and the y bonds are attached to adjacent carbon atoms on the ring structures; 
     each R, when present as a substituent, is independently selected from alkyl, alkoxy, aryloxy, aryl, aralkyl, alkaryl, alkoxyalkyl, cycloaliphatic, halogen, alkanoyl, alkanoyloxy, alkoxycarbonyl, carboxyl, cyano or formyl radicals; 
     n is a whole number in the range of 0-4 were Ar is phenyl; 0-6 where Ar is naphthyl; and 0-8 where Ar is phenanthryl or anthracenyl; 
     each R 1  and R 2  is independently selected from alkyl, aryl, aralkyl, alkaryl or cycloaliphatic radicals, or substituted derivatives thereof; 
     each R 3  and R 4  is independently selected from hydrogen and the R 1  substituents; 
     each of the above alkyl groups or moieties is straight or branched chain of 1-20 carbons; 
     each aryl group contains 6-10 rings carbons; 
     each cycloaliphatic group contains from 4-8 ring carbons; 
     each Y is independently selected from the elements, N, P, As, Sb and Bi; and 
     substituted derivatives include ethers, amines, amides, sulfonic acids, esters, hydroxyl groups and alkoxy groups. 
     The invention process comprises coupling two molecules of a reactant of the formula ##STR2## by maintaining a redox reaction system comprising said reactant, a polar aprotic solvent, a nickel compound and a reducing agent at a temperature and for a time sufficient to form the desired bidentate ligand.

This is a divisional of copending application Ser. No. 118,573, filed onNov. 9, 1987, now U.S. Pat. No. 4,879,008.

This invention relates to the preparation of bidentate ligands which areuseful, for example, in the formation of low pressure hydroformylationcatalysts.

BACKGROUND OF THE INVENTION

Bidente ligands have recently been shown to be very effective for thepreparation of organometallic catalysts, such as for example, lowpressure hydroformylation catalysts wherein the bidentate ligands arecoordinated with rhodium. While a variety of bidentate ligands areuseful for such chemical conversions as hydroformylation, theirsynthesis is often difficult, involving numerous reaction steps, one ormore of which give low product yields. The net result is that the targetbidentate ligands are obtained in low overall yields and are expensiveto prepare.

In order for bidentate ligands such as: ##STR3## to come into morewidespread use, efficient means for the preparation of such bidentateligands will need to be developed.

OBJECTS OF THE INVENTION

It is, therefore, an object of the present invention to develop improvedmethods for the preparation ofbis(dihydrocarbylphosphinomethyl)-biphenyl-type bidentate ligands.

This and other objects will become apparent from inspection of thedetailed description and claims which follow.

STATEMENT OF THE INVENTION

In accordance with the present invention, we have discovered thatbis(dihydrocarbylphosphino-methyl)-biphenyl-type compounds can beprepared by the reductive coupling of two molecules of a halogensubstituted aromatic phosphine. The resulting diphosphine compounds areuseful as bidentate ligands in combination with a wide variety of activemetal species. For example, when employed in combination with rhodium,the bis(dihydrocarbylphosphinomethyl)-biphenyl-type compounds preparedin accordance with the present invention are useful as components of lowpressure hydroformylation processes. Such catalyst systems produceunusually high proportions of normal (or unbranched) aldehydes fromα-olefins, e.g., n-butyraldehyde from propylene.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, there is provided a processfor preparing a bidentate ligand of the formula: ##STR4## wherein:

each Ar is independently selected from aromatic ring compounds having 6up to 14 carbon atoms, e.g., phenyl, naphthyl, phenanthryl andanthracenyl;

the x bonds and the y bonds are attached to adjacent carbon atoms on thering structures;

each R, when present as a substituent, is independently selected fromalkyl, alkoxy, aryloxy, aryl, aralkyl, alkaryl, alkoxyalkyl,cycloaliphatic, halogen, alkanoyl, alkanoyloxy, alkoxycarbonyl,carboxyl, cyano or formyl radicals;

n is a whole number in the range of 0-4 were Ar is phenyl; 0-6 where Aris naphthyl; and 0-8 where Ar is phenanthryl or anthracenyl;

each R₁ and R₂ is independently selected from alkyl, aryl, aralkyl,alkaryl or cycloaliphatic radicals, or substituted derivatives thereof;

each R₃ and R₄ is independently selected from hydrogen and the R₁substituents;

each of the above alkyl groups or moieties is straight or branched chainof 1-20 carbons;

each aryl group contains 6-10 rings carbons;

each cycloaliphatic group contains from 4-8 ring carbons;

each Y is independently selected from the elements N, P, As, Sb and Bi;and

substituted derivatives include ethers, amines, amides, sulfonic acids,esters, hydroxyl groups and alkoxy groups.

The invention process comprises maintaining a redox reaction systemcomprising a reactant of the formula: ##STR5## a polar, aprotic solvent,a nickel compound, and a reducing agent at a temperature suitable forcoupling for a time sufficient to form the desired bidentate ligand. Itis of note that no other reaction components are required to accomplishthe desired coupling reaction, e.g., no added ligand is required for thenickel component.

In a particular embodiment of the present invention, the bidentateligands prepared in accordance with the invention process are compoundsof the formula: ##STR6## wherein:

n is 0-4;

each R is independently selected from alkyl, alkoxy, aryloxy, aryl,aralkyl, alkaryl, alkoxyalkyl, cycloaliphatic, halogen, alkanoyl,alkanoyloxy, alkoxycarbonyl, cyano, carboxyl or formyl radicals;

each R₁ and R₂ is independently selected from alkyl, aryl, aralkyl,alkaryl or cycloaliphatic radicals, or substituted derivatives thereof;

each R₃ and R₄ is independently selected from hydrogen and the R₁substituents;

each of the above alkyl groups or moieties is straight or branched chainof 1-20 carbons, each aryl group contains 6-10 ring carbons, and eachcycloaliphatic group contains from 4-8 ring carbons;

each Y is independently selected from the elements N, P, As, Sb and Bi,with P being preferred; and

substituted derivatives include ethers, amines, amides, sulfonic acids,esters, hydroxyl groups and alkoxy groups.

In another particular embodiment of the present invention the bidentateligands prepared in accordance with the invention process are compoundsof the general formula: ##STR7## wherein:

the x bonds and the y bonds are attached to adjacent carbon atoms on thering structures;

each R when present as a substituent is independently selected fromalkyl, alkoxy, aryloxy, aryl, aralkyl, alkaryl, alkoxyalkyl,cycloaliphatic, halogen, alkanoyl, alkanoyloxy, alkoxycarbonyl, cyano,carboxyl, or formyl radicals;

each R₁ and R₂ is independently selected from alkyl, aryl, aralkyl,alkaryl or cycloaliphatic radicals, or substituted derivatives thereof;

each R₃ and R₄ is independently selected from hydrogen and the R₁substituents;

each of the above alkyl groups or moieties is straight or branched chainof 1-20 carbons, preferably 1-8 carbons, each aryl group contains 6-10ring carbons, and each cycloaliphatic group contains from 4-8 ringcarbons;

each Y is independently selected from the elements N, P, As, Sb and Bi,with P being preferred; and

substituted derivatives include ethers, amines, amides, sulfonic acids,esters, hydroxyl groups and alkoxy groups.

In yet another particular embodiment of the present invention, thebidentate ligands prepared in accordance with the invention process arecompounds of the general formula: ##STR8## wherein:

the x bonds and the y bonds are attached to adjacent carbon atoms on thering structure;

each R when present as a substituent is independently selected fromalkyl, alkoxy, aryloxy, aryl, aralkyl, alkaryl, alkoxyalkyl,cycloaliphatic, halogen, alkanoyl, alkanoyloxy, alkoxycarbonyl, cyano,carboxyl, or formyl radicals;

each R₁ and R₂ is independently selected from alkyl, aryl, aralkyl,alkaryl or cycloaliphatic radicals, or substituted derivatives thereof;

each R₃ and R₄ is independently selected from hydrogen and the R₁substituents;

each of the above alkyl groups or moieties is straight or branched chainof 1-20 carbons, preferably 1-8 carbons, each aryl group contains 6-10ring carbons, and each cycloaliphatic group contains from 4-8 ringcarbons;

each Y is independently selected from the elements N, P, As, Sb and Bi,with P being preferred; and

substituted derivatives include ethers, amines, amides, sulfonic acids,esters, hydroxyl groups and alkoxy groups.

Especially preferred compounds which can be prepared in accordance withthe invention process include:

2,2'-bis(diphenylphosphinomethyl)-1,1'-biphenyl (hereinafter, BISBI);

2,2'-bis(dibenzylphosphinomethyl)-1,1'-biphenyl;

2,2'-bis(phenylbenzylphosphinomethyl)-1,1-biphenyl;

2,2'-bis(disobutylphosphinomethyl)-1,1'-biphenyl;

2-(diphenylphosphinomethyl)-1-[2-(diphenyl-phosphinomethyl)phenyl]naphthalene;and

2,2'-bis(diphenylphosphinomethyl)-1,1'-binaphthyl.

The reductive coupling reaction is generally carried out at atemperature in the range of about 50° C. and 200° C., preferably betweenabout 110° C. and about 140° C.

Reaction pressure employed for the reductive coupling is not critical.Typically, the reaction is carried out at atmospheric pressure, althoughhigher and lower pressures can be employed.

The reducing agent metal is generally present with respect to the nickelcompound in a molar ratio in the range of about 5/1 up to 1,000/1,preferably in the range of about 10/1 up to 400/1, and most preferablyfrom about 25/1 to about 100/1, although higher or lower ratios may beused. Very low ratios, however, will typically result in incompletereaction and low yield.

It is also preferred that the ratio of polar, aprotic solvent (in mL)with respect to the reactant (halobenzyl phosphine; in moles) be in therange of about 100/1 up to 10,000/1, and most preferably in the range ofabout 200/1 up to 2,000/1. The molar ratio of nickel compound withrespect to the reactant (halobenzyl phosphine) should be in range ofabout 2/1 up to 100/1, preferably in the range of about 5/1 up to 40/1,and most preferably in the range of about 10/1 up to 30/1. While higheror lower ratios may be used, there are no practical reasons therefor.

Solvents suitable for use in the practice of the present invention arepolar (i.e., high dipole moment), aprotic solvents, such as, forexample, dimethylformamide, dimethylacetamide, N-methyl pyrrolidone,N,N-dimethyl benzamide, N-methyl piperidone, benzonitrile, and the like.

A wide range of nickel compounds are suitable for use in the practice ofthe present invention, so long as the nickel compounds employed areessentially water-free. The nickel (II) halide salts are a convenientsource of nickel as such compounds are readily available in anhydrousform. Those of skill in the art recognize that a wide variety of othernickel compounds can be used, e.g., nickel nitrates, sulfates,phosphates, oxides, carbonates, carboxylates, acetylacetonate and thelike, as well as Ni(O) complexes such as, for example,bis(1,5-cyclooctadienyl)nickel(O), nickel(O) tetracarbonyl, and thelike.

The nickel (II) halides are presently preferred because of their readyavailability in anhydrous form, and because the presence of halides inthe reaction mixture appears to promote the coupling reaction.

When halide-free nickel compounds are employed, it may be desirable toprovide a source of halide to the reaction mixture. A convenientsupplemental source of halide is an alkali metal halide, preferably asthe sodium or potassium halide. Up to about 200 moles of halide per moleof nickel will impart a beneficial effect on the coupling reaction, withabout 10 up to 80 moles of halide per mole of nickel being preferred. Ina most preferred embodiment, about 20 up to 50 moles of halide per moleof nickel will be added to the coupling reaction mixture.

The reducing agent employed in the invention process will have asufficient reducing potential to promote the reduction of Ni(II) toNi(O). Thus, any element with an electromotive force (EMF) more negativethan -0.25 V could be employed. Elements which satisfy this criterioninclude calcium, zinc, magnesium, manganese, sodium and lithium.Presently preferred elements are zinc, magnesium and manganese.

While the reducing agent employed in the practice of the presentinvention is preferably internal to the reaction system, those of skillin the art recognize that the known external reducing agent, anelectrochemical cell, can also be used. In such a system, conventionalE.M.F. values for the particular concentrations of the aryl halidereactant to be coupled, nickel compound and electrolyte, e.g.,tetrabutylphosphonium bromide, lithium bromide, etc., can be employed.The determinations of such E.M.F., component concentrations, bath sizeand the like can readily be carried out by those skilled in the art.

A typical useful electrochemical cell is ##STR9## Undivided cells mayalso be used. In carrying out such an electrochemical reaction in thelaboratory, the following parameters are exemplary for coupling2-chlorobenzyldiphenylphosphine (2-CBDP):

    ______________________________________                                        Bath size          1.0 L                                                      Dimethylformamide  500 mL                                                     2-CBDP             0.4 moles                                                  NiCl.sub.2         0.02 moles                                                 LiBr               0.3 N                                                      E.M.F.             -1.5 volts (relative to                                                       the Saturated                                                                 Calomel                                                                       Electrode)                                                 ______________________________________                                    

It is preferred to agitate the bath in known manner and to maintain theelectrochemical reaction mixture at a temperature suitable for producingthe coupled product. The temperature of the electrochemical reactionmixture is preferably maintained in the range of about 50° C. to 200°C., and most preferably in the range of about 110° C. up to 140° C.

In the reductive coupling reaction, the solvent employed is preferablydimethylformamide or dimethylacetamide, or mixtures thereof; the nickelcompound employed is preferably nickel chloride or nickel bromide, ormixtures thereof; and the reducing metal employed is preferably finelydivided, preferably powdered, zinc, magnesium or manganese, or mixturesof two or more thereof.

During the reductive coupling reaction, the concentrations of thevarious reactant materials and their ratios as set forth above willnecessarily change and it is preferred for continuous operations thattheir concentrations be maintained at least within the specified broadranges by addition of these reactants to the reaction system as isnecessary.

It is also noted with respect to the above stated reaction conditions,that the temperatures employed will be dictated to a degree by theparticular reactants employed, and also by the size and design of theequipment. For example, the thermal stability of these materials must beconsidered and any exotherm monitored to prevent degradation orexcessive side reactions. The pressure of the reductive couplingreaction systems need only be ambient, and lower or higher pressuresgive no significant enhancement to the reaction and typically are notwarranted.

In regard to the isolation and work up of the coupled product, theprocedure generally involves the following sequence of steps: aqueousquench, filtration, aqueous washes, distillation or concentration, andrecrystallization. The crude product obtained by this work up typicallycontains about 500 to 750 ppm of nickel, in such forms as polymericnickel phosphine complexes and discrete monomeric complexes with thebidentate ligand, both types of complexes containing nickel in the 0 and+2 oxidation states.

One of skill in the art can readily determine various means to reducethe nickel content of the coupling reaction mixture, e.g., extraction,recrystallization, chromatographic methods and the like. When extractionis employed, extracting solutions useful for this purpose includecaustic/-cyanide, ethylenediaminetetraacetic acid, ammonium hydroxide,dimethyl glyoxime, concentrated hydrochloric acid, oxalic acid, and thelike.

The presently preferred technique for separating the nickel complexesfrom bidentate ligand is by recrystallization from a suitable solvent,such as, for example, acetone, methyl ethyl ketone, methanol/-acetone,ethanol/acetone, and the like.

The reactant which is subjected to reductive coupling in accordance withthe present invention can be prepared in a variety of ways. For example,a metal halide reagent of the formula: ##STR10##

the x bonds and the y bonds are attached to adjacent carbon atoms on thering structures;

and wherein M is selected from the group consisting of Li, MgX, Na, K,Cd, Zn and Ca can be contacted with a compound of the formula: ##STR11##wherein X is a halogen, under conditions suitable to form the desiredreactant.

The reaction for preparing the desired reactant is typically carried outin the presence of a solvent such as diethyl ether, tetrahydrofuran(THF), THF/toluene mixtures, aprotic dialkyl ethers, ethylene glycoldialkyl ethers, particularly ethylene glycol dimethyl-, dipropyl-, anddibutyl-ethers; most preferably diethyl ether. The reaction is carriedout at a temperature in the range of about 0° C. up to 60° C.,preferably at about the reflux temperature of the solvent. Reactionpressure is not critical, and is preferably about one atmosphere.

The molar ratio of the metal halide reagent to the diorgano-Group Vhalide reactant can vary widely, with a ratio of essentially 1/1typically being employed since no excess of either is necessary.

Alternatively, the reactant can be prepared by contacting a halobenzylcompound of the formula: ##STR12## wherein X¹ is a halogen orappropriate leaving group, e.g., tosylate, mesylate, brosylate, and thelike; and

the x bonds and the y bonds are attached to adjacent carbon atoms on thering structures;

with a diorganometallo-Group V compound of the formula: ##STR13##wherein M' is selected from the group consisting of lithium, sodium,potassium, magnesium, calcium, zinc and cadmium.

This contacting is preferably carried out in an anhydrous, aproticsolvent, such as, for example, an ether (e.g., diethyl ether,tetrahydrofuran), aromatic hydrocarbon (e.g., toluene, xylene), as wellas mixtures of any two or more thereof.

It is preferred, for ease of manipulation, that the metallo moiety bepreformed in a separate reaction vessel, and thereafter contacted withthe halobenzyl compound.

One example of this synthetic approach is the reaction of a strong base,such as n-butyl lithium, with a secondary organophosphine, e.g.,diphenyl phosphine.

Alternatively, the metallo moiety can be formed by the reductivecleavage of a tertiary organophosphine employing dissolving metalreactions. For example, sodium diphenylphosphide can be produced bytreating chlorodiphenylphosphine in toluene with sodium metal; orlithiodiphenylphosphine can be prepared by treating triphenylphosphinein tetrahydrofuran with lithium metal.

As another alternative, a halobenzyl-Group V oxide of the structure:##STR14## can be prepared, then reduced to give the desired reactant.This is an especially attractive alternative when the desired precursorcompounds are not readily available. Thus, dialkyl halobenzylphosphinescan readily be prepared in this manner. For example, one equivalent ofdiethyl phosphite can be treated with three equivalents ofbenzylmagnesium chloride to produce the magnesium salt ofdibenzylphosphine oxide. The secondary phosphine oxide can also beformed by the reaction of a strong base (e.g., n-butyl lithium) with asecondary phosphine oxide. Reaction of the resulting organometalliccompound with 2-chlorobenzyl chloride producesdibenzyl-(2-chlorobenzyl)phosphine oxide which can then be reduced tothe phosphine with an appropriate reducing agent, such as, for example,lithium aluminum hydride or trichlorosilane.

The following examples will further illustrate the invention:

EXAMPLE 1 Preparation of 2-Chlorobenzyldiphenylphosphine

(a) To a suspension of magnesium turnings (15.50 grams, 0.636 mole) indiethyl ether (500 mL) under nitrogen in a three-necked flask equippedwith a reflux condenser, addition funnel, and mechanical stirrer wasadded a solution of 2-chlorobenzylchloride (93.0 grams, 0.578 mole) indiethyl ether (100 mL). The addition was made at a rate such that agentle reflux was maintained throughout the addition. Upon completion ofthe addition, the reaction was heated to reflux for an additional 0.5hour to give a solution of the desired Grignard reagent. Thispreparation of the Grignard reagent is typical and other conditions forsuch preparation are known to the art and may be employed in connectionwith the present invention.

(b) Chlorodiphenylphosphine (115.84 grams, 0.525 mole) in diethyl ether(200 mL) was added dropwise with vigorous stirring to the above solutionof the Grignard reagent at a rate such that a moderate reflux wasmaintained. Upon completion of the addition, the reaction was refluxedfor 1 hour, cooled to ambient temperature, and quenched withconcentrated HCl (50 mL) in 400 mL of water. The quench was initiallyhighly exothermic and required a careful dropwise addition. After allthe solids were digested, the layers were separated and the organiclayer washed with water (2×200 mL). The wash mixtures were filtered toremove residual solid contaminants, the layers separated, and the finalorganic layer stripped to dryness to give 167 grams of crude product asa crystalline solid. This product was analyzed and found to besufficiently pure to use without further purification.

EXAMPLE 2 Preparation of BISBI From 2-ChlorobenzyldiphenylphosphineUsing Magnesium as the Reducing Metal

To a nitrogen purged 50-mL flask was charged2-chlorobenzyldiphenylphosphine (6.21 grams, 0.02 mole), magnesiumpowder (1.46 grams of -325 mesh, 0.06 mole), anhydrous nickel chloride(0.13 gram, 0.001 mole), and dimethylformamide (20 mL). The reactionsystem was heated to 120° C. and held at that temperature for 8 hours.The mixture was then cooled to room temperature and quenched by theaddition of ether (50 mL) and water (50 mL). The solids were removed byfiltration and the layers separated. The organic phase was successivelywashed, with layer separation, with 1N hydrochloric acid (1×50 mL), with5 percent sodium bicarbonate (1×25 mL), and with saturated NaCl solution(1×50 mL). The final organic layer was stripped to give the crudeproduct which was then dissolved in acetone (50 mL) and filtered toremove polymeric nickel species. The acetone solution was concentratedto 10 mL, diluted with methanol (25 mL), chilled to 0° C. and theproduct filtered therefrom. The product was dried under vacuum to give3.31 grams (60 percent of theory) of BISBI which was pure by phosphorus31 NMR.

EXAMPLE 3 Preparation of BISBI From 2-ChlorobenzyldiphenylphosphineUsing Zinc as the Reducing Metal

To a three-neck, one-liter flask equipped with reflux condenser,thermometer, and stirring bar was added 2-chlorobenzyldiphenylphosphine(114.9 grams, 0.37 mole), anhydrous nickel bromide (4.05 grams, 0.0185mole), zinc (72.57 grams of -325 mesh powder, 1.11 mole), anddimethylformamide (300 mL). The reaction mixture was heated withstirring to 120° C. whereupon the reaction exothermed to approximately150° C. The exotherm was controlled by external cooling. After theinitial exotherm subsided, the reaction mixture was maintained at 120°C. for 2 hours and the disappearance of starting material was monitoredby gas chromatography. The reaction mixture was cooled to roomtemperature, ether (300 mL) and water (200 mL) were then added. Thereaction mixture was then filtered, mainly to remove the excess zinc.The ether and aqueous layers were separated and the initial organicether layer or phase was washed successively, with separation, withwater (200 mL), with 5 percent HCl (200 mL), with 5 percent sodiumbicarbonate (200 mL), and again with water (200 mL). All of the aqueousphases except the bicarbonate phase were combined and backwashed with a1/1 mixture of hexane and ether (200 mL total). The separated organicphase was then combined with the initial organic phase. The combined(final) organic phase was concentrated to give a crude product whichsolidified and was triturated in hexane (400 mL) and ether (30 mL) togive a powdery yellow solid. The solid was dissolved in hotethanol/acetone (600 mL of a 5/1 mixture), filtered hot, and allowed toslowly cool to room temperature. This crystallization mixture was thencooled to 0° C., filtered and the cake rinsed with ice cold ethanol togive a white solid which was dried in vacuo to give 74.5 grams of BISBI.This product was analyzed and found to contain 18 ppm of nickel. Theliquor was concentrated and chilled to obtain a second crop of crystalsamounting to 5.5 grams. The overall yield of BISBI was 80.0 grams whichis 80 percent of the theoretical yield.

EXAMPLE 4 2,2'-Bis(dibenzylphosphinomethyl)-1,1'-biphenyl Dioxide

Dibenzylphosphine oxide (6.93 grams, 30.1 mmol) and THF (100 ml) wereplaced in a 300-ml three-necked flask and cooled at -40° C. undernitrogen. n-Butyllithium (18.84 ml of a 1.6M solution in hexane, 30.1mmol) was added dropwise from an addition funnel over about 10 minutesand the resulting yellow solution was stirred for one hour at -30° C. to-35° C. 2,2'-Bis(bromomethyl)-1,1'-biphenyl (5.00 grams, 14.7 mmol) inTHF (20 ml) was added dropwise to the cold solution. When the additionwas complete, the solution was warmed to room temperature and was thenheated at reflux for 1.5 hours. Saturated aqueous NH₄ Cl was added andthe layers were separated. The aqueous layer was extracted twice withdiethyl ether. The combined organic solution was washed with saturatedaqueous NaCl. The organic solvent was evaporated on a steam bath under astream of nitrogen to give a light brown solid. The product wasrecrystallized from acetone to give a first crop of 3.57 grams (38%yield) of white solid, melting point 203° to 205° C. No attempt was madeto recover a second crop.

¹ H NMR (CDCl₃): δ2.07-3.08 (complex, 12H, benzylic); 6.57-7.47(complex, 28H, aromatic). ³¹ p NMR (CDCl₃): δ-43.

EXAMPLE 5 2,2'-Bis(dibenzylphosphinomethyl)-1,1'-biphenyl

Chlorotrimethylsilane (4.1 ml, 32.2 mmol) was added to lithium aluminumhydride (1.22 grams, 32.2 mmol) in THF (20 ml) at -72° C. The mixturewas removed from the cold bath, stirred two hours, and then cooled againat -35° C. A suspension of the above2,2'-bis(dibenzylphosphinomethyl)-1,1'-biphenyl dioxide (3.40 grams,5.32 mmol) in THF (45 ml) was added by cannula. The mixture was stirred0.5 hour at -30° C., then overnight at room temperature. The reactionmixture was cooled in an ice bath and quenched by the successive,dropwise addition of water (1.2 ml), 15% aqueous NaOH (1.2 ml) and water(3.6 ml). The resulting mixture was filtered, and the solid was washedwith diethyl ether. The filtrate was evaporated on the steam bath undera stream of nitrogen. The residual solid was heated in ethanol, thencooled and filtered to give 2.00 grams (62% yield) of white solid,melting point 163° to 167° C.

¹ H NMR (CDCl₃): δ2.43 (s, 12H, benzylic); 6.50-7.17 (complex, 28H,aromatic). ³¹ P NMR (CDCl₃): δ+9.5.

It is thus seen that the present invention provides a new syntheticroute to economically valuable organophosphine ligands, particularly,2,2'-bis(diphenylphosphinomethyl)-1,1'-biphenyl (BISBI) by the reductivedimerization of 2-halobenzyldiorganophosphines. The2-halobenzyldiorganophosphines can be prepared by a variety oftechniques, e.g., reaction of the Grignard reagent of 2-halobenzylhalide with halodiorganophosphine using standard procedures. In ExampleI, the intermediate reactant was prepared in sufficiently pure form thatit could be used without further purification. Another advantage of thisroute is that the product organophosphine ligand is readily available inhigh yield.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

I claim:
 1. A process for preparing a bidentate ligand of the formula:##STR15## wherein: each Ar is independently selected from aromatic ringcompounds having 6 up to 14 carbon atoms;the x bonds and the y bonds areattached to adjacent carbon atoms on the ring structures; each R, whenpresent as a substituent, is independently selected from alkyl, alkoxy,aryloxy, aryl, aralkyl, alkaryl, alkoxyalkyl, cycloaliphatic, halogen,alkanoyl, alkanoyloxy, alkoxycarbonyl, carboxyl, cyano or formylradicals; n is a whole number in the range of 0-4 where Ar is phenyl;0-6 where Ar is naphthyl; and 0-8 where Ar is phenanthryl oranthracenyl; each R₁ and R₂ is independently selected from alkyl, aryl,aralkyl, alkaryl or cycloaliphatic radicals, or substituted derivativesthereof, wherein the substituted derivatives are selected from ethers,amines, amides, sulfonic acids, esters, hydroxyl groups or alkoxygroups; each R₃ and R₄ is independently selected from hydrogen and theR₁ substituents; each of the above alkyl groups or moieties is astraight or branched chain of 1-20 carbons; each aryl group contains6-10 ring carbons; each cycloaliphatic group contains from 4-8 ringcarbons; and each Y is independently selected from the elements N, P,As, Sb and Bi; said process comprising:(i) contacting a reagent havingthe structure: ##STR16## wherein X is a halogen; M is selected from thegroup consisting of Li, MgX, Na, K, Cd, Zn and Ca; and the x bonds andthe y bonds are attached to adjacent carbon atoms on the ringstructures; with a compound of the formula: ##STR17## wherein X' ishalogen or a suitable leaving group under conditions suitable to form areactant of the formula: ##STR18## then (ii) maintaining a redoxreaction system comprising(a) said reactant, (b) a polar, aproticsolvent, (c) a nickel compound, and (d) a reducing agent, which has asufficient reducing potential to promote the reduction of Ni(II) toNi(O), at a temperature and for a time sufficient to form said ligand.2. The process of claim 1 wherein the reactant has the formula:##STR19##
 3. The process of claim 1 wherein the reducing agent isselected from finely divided Zn°, Mg° or Mn° and is present with respectto the nickel compound in a molar ratio of reducing agent to nickelcompound in the range of about 5/1 up to 1000/1.
 4. The process of claim1 wherein said redox system is maintained at a temperature in the rangeof about 50 up to 200° C.
 5. The process of claim 1 wherein said redoxsystem is maintained at a temperature in the range of about 110° up to140° C.
 6. The process of claim 1 wherein M is MgX.
 7. The process ofclaim 6 wherein X is chlorine, R₃ and R₄ are H, Ar is phenyl, n is zero,and each R¹ and R² is independently selected from the group consistingof phenyl, benzyl, and alkyl radicals having 1-6 carbon atoms.
 8. Theprocess of claim 6 wherein the reactant has the formula: ##STR20## 9.The process of claim 1 wherein the molar ratio of the reducing agent tothe nickel compound falls within the range of about 10/1 up to 400/1,and the molar ratio of the reactant to the nickel compound falls in therange of about 2/1 up to 100/1.
 10. The process of claim 1 wherein themolar ratio of the reducing agent to the nickel compound falls withinthe range of about 25/1 up to 100/1, and the molar ratio of the reactantto the nickel compound falls in the range of about 5/1 up to 40/1. 11.The process of claim 1 wherein the reducing agent is an electrolyticcell.